In industry the use of metal products manufactured by compacting and sintering iron-based powder compositions is becoming increasingly widespread. A number of different products of varying shapes and thickness are being produced. One processing technique for manufacture the products from the base powders is to charge the powder into a die cavity and compact the powder under high pressures. The obtained compact is then removed from the die cavity and sintered.
The quality requirements of the products are continuously raised, and in this context one important factor is that the manufactured products have high and consistent density. Much effort is put into research to develop such products and one field within this research concerns lubrication, which is used i.a. to avoid excessive wear on the die cavity during compaction. Lubrication is accomplished by spraying a liquid dispersion or solution of the lubricant onto the die cavity surface (external lubrication) or by blending a solid lubricant powder with the iron-based powder (internal lubrication). In some cases, both lubrication techniques are utilized.
The use of external, die wall lubricants can reduce or eliminate the need for an internal lubricant, but problems accompany external lubrication techniques. First, the film thickness within the die cavity has a tendency to vary, and the lubricant dispersion is known to drip out of the die cavity during processing. Also, aqueous dispersions are a source of rust formation on the die cavity. Another problem is that various external lubricant compositions are not necessarily sufficient to adequately lower ejection forces, especially at higher compaction pressures. Finally, as a technique, the die wall lubrication does not permit high productivity in comparison with internal lubrication.
Lubrication by means of blending a solid lubricant into the iron-based powder composition has also disadvantages. One problem is that the lubricant generally has a density of about 1-1.2 g/cm3, as compared with the density of the iron-based powder, which is about 7-8 g/cm3. Inclusion of the less dense lubricant in the composition lowers the green density of the compacted part. Second, internal lubricants are generally not sufficiently effective for reducing the ejection pressures when manufacturing parts having part heights in excess of about 2.5-5 cm. Another problem is, when the particles of internal lubricant burn off during sintering, pore spaces can be left in the compacted part, providing a source of weakness for the part. Many presently used lubricants also have the disadvantage of requiring high energies for ejecting the green compact from the die.
Another disadvantage with presently used lubricants is that they often include zinc stearate. This is due to the fact zinc stearate imparts good flow properties to metal powder compositions including this stearate. In reducing atmospheres, the zinc oxide remaining after initial decomposition of the stearate is reduced to zinc, which readily volatilises because of its low boiling point. Unfortunately, on contacting the cooler parts of the furnace or the outside atmosphere, the zinc tends to condense or reoxidise. A consequence of reactions is that the production has to be interrupted as the furnace has to be cleaned regularly.
The problems associated with zinc stearate can be avoided by the use of completely organic materials such as waxes. The wax most widely used in powder metallurgy is ethylene-bisstearamide, EBS, (available under the name Acrawax™ C or Licowax™). This material has a high melting point (140° C.) but it burns off at relatively low temperatures and leaves no metallic residue. The most serious disadvantage is its poor flow behaviour in metal powders.
The present invention is particularly directed to iron based compositions with internal lubrication and wherein the lubrication is provided by a new lubricant composition including glyceryl stearate.
An additional field of application of the glyceryl stearate compositions or mixtures according to the present invention is as binders for pulverulent additives to the metal powder which makes it possible to produce non dusting powder mixtures that are also free from segregation.
Glyceryl stearate has been mentioned in connection with iron-based metal powders for the PM-industry in U.S. Pat. No. 5,518,639 and the related U.S. Pat. No. 5,538,684 which discloses lubricant compositions containing a solid phase lubricant, such as graphite, molybdenum disulfide, and polytetrafluoroethylene in combination with a liquid phase lubricant that is a binder for the solid phase lubricant. The binder can be chosen from various classes of compounds including polyethylene glycols, polyethylene glycol esters, partial esters of C3-6 polyhydric alcohols, polyvinyl esters, and polyvinyl pyrrolidones. The binder is solubilized in an organic solvent. This lubricant composition is applied to the surface of a die cavity prior to compaction of the metal powder composition. The glyceryl stearate is thus known use as a binder in connection with external lubrication and, in contrast to the lubricant according to the present invention, it is not mixed with the iron-based powder and optional additives before the compaction.
Furthermore, the U.S. Pat. No. 5,432,223 discloses that glyceryl stearate can be used as a plastiziser in polyvinylpyrrolidone, which is a binding agent which may be used in metal powder compositions.
Another patent which mentions glyceryl stearate in connection with metal powders is the U.S. Pat. No. 6,187,259. In this patent glyceryl stearate is mentioned together with a wide variety of other substances as an agent for providing hydrofobicity in rare earth alloy powders for the production of granules.
The U.S. Pat. No. 5,641,920 mentions the use of glyceryl monostearate as a plasticizer/compatibilizer in powders for injection moulding. In an article “Thermal extraction of binders and lubricants in sintering” by German in Advances in Powder Metallurgy & Particulate Materials, 1996 glyceryl monostearate is also mentioned.
In the recently published WO 03/015962 glycerol monostearate is mentioned in lubricant systems in combination with different guanidine compounds. The systems actually tested in combination with metal powders do however not include glyceryl stearate. No beneficial effects with guanidine compounds have been observed in connection with the present invention, a reason why such guanidine compounds are not included in the lubricant/binder system according to the present invention.